SAN FRANCISCO — Voter support for legal marijuana in Washington and Colorado is spurring similar campaigns in California and three other states that together may bring pot within lawful reach of nearly one in five Americans.

Advocates are seeking the signatures of registered voters in California, Arizona, Oregon and Alaska, with a combined population of 49 million, to put the question on ballots in 2014. Colorado and Washington last year legalized marijuana for 12.1 million people.

“Because of Colorado and Washington, it’s created a cannabis tidal wave across the country,” Mike Jolson, 45, a legalization activist in Santa Cruz, said by telephone. “We want to capitalize on this wave.”

Washington and Colorado became the first U.S. states to legalize recreational marijuana through referendums last November, defying federal law that has prohibited pot since the 1930s. In August, the U.S. Justice Department said it wouldn’t challenge the states, opening the door for others.

In Washington state, regulators are finalizing rules for growing, processing and selling marijuana ahead of a Dec. 1 deadline to begin issuing licenses. In Colorado, which has finished setting its rules, voters will decide next month whether to tax retail sales at rates of as much as 25 percent.

“Their success in Colorado was very inspiring, and I thought it would be a good time for us to try here,” said Dennis Bohlke, a computer programmer from Phoenix who said he modeled the Arizona initiative after the Colorado measure.

He has to collect 259,213 valid signatures by July 3 to add his measure to the November 2014 ballot.

In California, more than half of residents support legalizing marijuana, according to a poll by the nonpartisan Public Policy Institute of California in San Francisco — 52 percent of all adults. Counting only likely voters, the figure is 60 percent. The telephone survey of 1,703 residents was conducted Sept. 10-17 and had a margin of error of plus or minus 3.7 percentage points for all residents, and 4 percentage points for registered voters.

Because California is the largest state by population, the campaign for legalization needs 504,760 signatures by Feb. 24 to qualify. Jolson said he wants to get volunteers out in the street to collect signatures.

In Alaska, one of the least-populated states, just 30,169 signatures are needed before the legislature goes into session in January, Timothy Hinterberger, 57, a sponsor of the initiative, said by telephone. More than 20,000 have been collected so far, he said. If successful, the question would be added to the primary election ballot in August.

“With the passage of time, people who are now regular voters have a lot more experience with cannabis and people who are cannabis users,” Hinterberger said.

A majority, or 52 percent, of Americans favor legalizing marijuana use, compared with 45 percent who say it should remain illegal, according to a survey by the Pew Research Center released in April. Young people are the most supportive, the survey showed.

Deputy U.S. Attorney General James Cole said in an August memo to prosecutors that the federal government wouldn’t intervene in the formation of a regulatory structure to oversee recreational marijuana in Colorado and Washington, so long as they prevented out-of-state distribution, access to minors, drugged driving and revenue from going to gangs and cartels.

The Justice Department’s decision “certainly does accelerate change in public opinion and makes us more optimistic about our chances in 2014,” said Dan Riffle, federal policy director at the Marijuana Policy Project, a Washington-based advocacy group. “It makes us more optimistic that the federal government won’t be interfering with these laws when we pass them.”

In addition to recreational use, there are efforts to expand the 20 states that allow medical marijuana, Riffle said.

Ballot proposals to legalize medical marijuana use are being circulated in six states: Arkansas, Florida, Idaho, Nebraska, Utah and Wyoming, according to Allen St. Pierre, executive director of the National Organization for the Reform of Marijuana Laws, a Washington-based group that advocates legalization.

Two pot measures are being circulated for signatures in Oregon, where a referendum to legalize recreational pot use failed last year, 53 percent to 47 percent.

One measure to legalize marijuana use requires 116,284 signatures to qualify for the November 2014 ballot, while a second setting up an Oregon Cannabis Commission to regulate its growth and sale requires 87,213 signatures, according to the state. Paul Stanford, 53, of the Hemp & Cannabis Foundation, says he has until July 7 and has collected about 15,000 signatures for each measure.

“We’d like to see Oregon’s economy take advantage of this new market, which would be an economic boon,” Stanford said by telephone.

Marijuana legalization could generate about $8.7 billion annually in tax revenue for federal, state and local governments, according to a 2010 report by Jeffrey Miron, who teaches economics at Harvard University in Cambridge, Massachusetts.

Municipal governments are also weighing in. In Portland, Maine, voters next month will consider a measure to allow possession of up to 2.5 ounces of marijuana for adults who are at least 21.

Some marijuana proponents are holding off until the 2016 presidential election, when voter turnout is typically higher.

Chris Lindsey, an attorney from Missoula, Mont., and a legislative analyst at the Marijuana Policy Project, said he’s working toward adding a recreational-marijuana proposal to his state’s ballot in 2016.

“Elections that involve presidential races tend to bring out a younger set of voters and we think we’ll probably benefit from having younger voters,” Lindsey said by phone.

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